Select particular portion of an object
The following code uses local vertex coordinates: 
For global coordinates you just multiply with "matrix_world"
Take a look also at the MeshVertex struct here:
https://docs.blender.org/api/current/bpy.types.MeshVertex.html

import bpy

mesh = bpy.context.object.data

bpy.ops.object.mode_set(mode='EDIT')
bpy.ops.mesh.select_all(action='DESELECT') 
bpy.ops.object.mode_set(mode='OBJECT')

for v in mesh.vertices:
    #the location of a vertex is in MeshVertex.co as a Vector3
    #Therefore you can filter by comparing via index
    #here the z-Axis is beeing measured
    if v.co[2] &gt; 0:  
        print(f"v is selected: {v.co}")
        v.select = True